The scale on a map is 9cm : 6km. If the distance between two cities is 24km, how far apart in cm are the two cities on the map?
Solution: The scale means that every 9cm on the map corresponds to an actual distance of 6km. An actual distance of 24km is the same as 4 $\cdot$ 6km. The distance between the two cities on the map is 4 $\cdot$ 9cm, or 36cm.